Filters are widely employed to modify the frequency response of electronic circuits. Filters typically use pass or stop bands to modify the frequency response of a circuit by selectively transmitting or attenuating one or more frequencies within a spectrum. Filters may exhibit low pass, high pass, band pass, and band rejection attributes.
Wireless communications has greatly crowded the electromagnetic spectrum. Such signal congestion has increased the demand for high performance electronic filters. In particular, filters that function at microwave frequencies are particularly important in wireless communications. Designing passive RF components in the microwave region is, however, particularly challenging. For wireless applications, notch filters that have a narrow band pass region are particularly useful. Competent notch filters exhibit accurate frequency selectivity and low insertion losses. The issue of insertion losses is of particular concern in portable wireless devices that rely on batteries for power.
Electromagnetic Band Gap (“EBG”) structures offer an expedient solution to the wireless communications demand for high performance notch filters. EBG structures function as a filter by exploiting their inherent electromagnetic band gap. At frequencies outside of the band gap, EBG structures pass signals. However, at frequencies that are within the band gap, EBG structures block the transmission of the signal.
The band gap behavior of EBG structures, also commonly referred to as photonic crystals, arises from the periodicity of the crystal lattice that forms the EBG structure. There are a variety of ways to fabricate the lattices that form EBG structures. One such method includes forming periodic inclusions in a dielectric matrix. Another method is to form a lattice of metal balls within a dielectric matrix.
Whatever method is used to create the EBG structure, the formation of the electromagnetic band gap arises in much of the same way as it does in semiconductor materials. When electromagnetic waves propagate through a periodic structure or array, Bragg diffraction creates destructive interference between the waves at particular frequencies. This Bragg diffraction gives rise to the band gap of the structure or material. EBG structures exhibit a characteristic band gap that has a center frequency related to the lattice constant of the periodic array. Specifically, the center frequency of this characteristic band gap is proportional to the speed of light divided by twice the lattice constant multiplied by the square root of the dielectric constant of the embedding medium.
EBG structures are commonly used in optical communications devices. For optical applications, EBG structures are sized to enable on-chip integration. For the characteristic band gap to occur at optical frequencies, the overall EBG structure lattice should have a size on the order of microns. The high demands placed on filters for wireless communications applications has generated interest in the use of EBG structures at the microwave portion of the electromagnetic spectrum. However, for the characteristic band gap to occur at microwave frequencies such as 10 GHz, for example, the overall EBG structure lattice needs to have a relatively large size, on the order of centimeters. This relatively large size makes the use of EBG structures for microwave communications, particularly with portable wireless devices, problematic.
Even so, the use of EBG structures for wireless communications filters has numerous advantages. First, EBG filters have low losses, making them ideal for high quality radio frequency (RF) applications. In addition, the low cost of manufacturing EBG structures makes them highly competitive with other components. Consequently, there is a need to develop smaller EBG structures that are useful in microwave communications applications.